1. Field of Invention
This invention relates to the surgical cutting of bone cores.
2. Description of Prior Art
The process of cutting cores from bone is a surgical procedure seen more frequently in recent years. Cores are cut and removed from bone when it is required to gain access to the inside of the bone such as the medullary canal or the inside of the skull. Access may be required to create a window for therapy. Another reason to cut a bone core is to obtain a bone specimen for diagnostic purposes. A further reason to cut a bone core is to harvest bone for grafting. Still another reason to core bone is to insert something into the bone, such as an item of hardware. The removed core is sometimes replaced after surgery, when it is used to close the bone (the core becomes a plug) after the procedure is completed. In some cases, the removed core is morselized and thereafter used as bone graft. The core, therefore, must not be damaged by heat and it should have relatively smooth cut surfaces. It is desirable in all cases to minimize invasion of soft tissue and reduce trauma to the core and to the surrounding bone. These criteria are very important. Former techniques and hardware employed in cutting cores from bone are known to cause trauma to the tissue. The hardware previously available caused damage by tearing both the soft and the bony tissue. These early coring cutters (also known as trephines) have significantly pointed teeth. An example of one such cutter was the Trephine for iliac crest bone graft cutting, marketed by Zimmer Medical, Inc. The teeth of some coring cutters had a positive rake angle; that is, the surface of a tooth was at less than 90 degrees to the surface being cut. The acute angle of attack caused the tooth to dig in to the tissue. Additionally, the clearance angle (that angle behind the tooth, in relation to the surface being cut) was very high, allowing too quick advancement of the coring cutter as it revolved. Because of such tooth geometry, previous coring cutters were also difficult to control. These pointed teeth tend to dig into and grab and tear tissue. The pointed teeth digging into the soft tissue and even the bony tissue grabbed and pulled the cutter into the bone. The hardware of this invention is designed to provide a controlled advance of the cutter as it is revolved. Such controlled cutter advance prevents ‘digging in’ of the cutter and promotes smooth, controlled cutting. Another disadvantage in the pointed cutting surfaces of earlier technology is that the teeth are prone to premature wear at the cutting edges, thus becoming dull after little use. When dull, the teeth do not cut cleanly; they tear, rather than cut, both soft and bony tissue. The Cutter of this invention is designed stay sharp longer and therefore to cut cleaner. In addition to the physical design, the material technology employed in the cutter of this invention enables the Cutter to achieve a sharper edge and then maintain a sharper cutting action. Another problem with former technology was that some damage to the bone was caused by the generation of excessive heat. The friction created by dull cutting edges causes heating of the bone; both the parent bone and the cored bone. Such heating leads to osteonecrosis (bone death). Therefore, using hardware and methods available hereto-fore has resulted in the harvest of bone cores having impaired viability. Because of the materials from which this cutter is made, it demonstrably cuts more cleanly for an extended period of time with minimum friction. Reduced friction means reduced heating of the parent bone and of the core, resulting in more viable bone plugs. Further, with previous bone coring cutters, there was excessive friction on the bone by the cutter body, leading to increased temperatures. This friction lead to heating. The hardware of this invention is designed to reduce heating by reducing friction between the body of the Cutter and bone. The Cutter of this invention has both internal diameter and external diameter relief to allow freedom from friction between the shank and the bone. Additionally, by design, the Cutter of this invention conducts heat away from the cutting areas. Metal is a better heat conductor than is bone. The wide, long ramps behind the cutter faces of this Cutter conduct more heat than the slim pointed cutting faces of former cutters. Therefore the cutting sites remain cooler. Because of the design, the Cutter of this invention advances with easily controlled action without digging and tearing. This Cutter is sharper when new and stays sharper for longer periods of time. The Cutter of this invention creates less heat, and conducts heat away from the bone than did former cutters. The advantages of this Cutter result in cleaner, straighter holes and cleaner, cooler, more viable bone and bone cores.